Thursday, January 31, 2008

There are a large number of systems that single-celled organisms use to move or swim. Those systems that work by sticking something large out of the cell and moving it are often called "flagella." The word flagellum

was originally the latin word for whip.

Although the same term is used, there are three (known) kinds of "flagella" that are very different in detail. They are often confused because terminology is often used inconsistently.

Bacteria have flagella. The motor is at the base and they rotate. They are always called "flagella" or "bacterial flagella".

Archaea (also known as archaebacteria) have flagella. The motor is at the base and they rotate. But, despite early assumptions of relatedness to bacterial flagella based on these similarities, they are very different in detail. They are always called "flagella" or "archaeal flagella".

Eukaryotes have a tubulin-based organelle that does not rotate. Instead, this organelle bends all along its length, powered by hundreds of dynein motor proteins. This organelle is called variously a flagellum, cilium, or undulipodium, or sometimes other names. This leads to a great deal of confusion for people new to the topic. Usage typically works like this:

Most microbiologists call it a "flagellum" if the eukaryotic cell has one or a few long appendages (such as sperm cells), and call it a "cilium" if the cell has many shorter appendages (such as a paramecium)

Some people point out that eukaryotic flagella and cilia have fundamentally the same 9+2 tubulin structure (usually), dyneins, etc., and are really essentially the same thing. They propose that the word "cilium" be used for both kinds of structures, and that "flagellum" be reserved for the prokaryote organelles.

This is the position and usage of Cavalier-Smith, and this usage is followed by Behe in Darwin's Black Box - Lynn Margulis and her followers (relatively few but published widely) call the eukaryotic structure a "undulipodium", both in order to distinguish it from the prokaryote organelles and to emphasize their symbiotic theory for the origin of the organelle.